1. Field of the Invention
This invention relates to the art of electric welding, and particularly to a control system for automatically isolating a welding electrode holder from the welding power source during a no-load, high voltage condition.
2. General Description of the Prior Art
Electric arc welding has been practiced for about 50 years. It requires a relatively high powered current source, one capable of providing ample current to melt welding electrodes that range from 1/16 to 5/16 inch in diameter at an operating voltage of 20 to 40 volts, AC or DC. It is also necessary that the power source provide an open circuit (no load) voltage significantly higher than the load voltage, typically 70 to 100 volts in order to readily strike an arc and provide the short circuit recovery drive necessary for stable welding.
The general arrangement of equipment for manual electric arc welding in common use is a welding power source that has two terminals for the welding circuit, with one of the terminals connected via a power cable to the work piece to be welded, and the other terminal connected via a flexible power cable to an electrode holder. The electrode holder is, in essence, an insulated metal clamp specially designed for holding a welding electrode. The clamp portion is supported by an insulated handle which is gripped by the welder. A welding electrode is a round metal rod that may be from 1/16 to 5/16 inch in diameter and range from 9 to 18 inches long. Approximately one inch of the electrode at one end is bare for good electrical contact when it is inserted into the jaws of the electrode holder. The remainder of the electrode is coated with various chemical formulations. The coatings usually consist of fluxing agents, arc stabilizers, varying amounts of metal powders, and proprietary substances. The coatings exhibit little, if any, insulating effect; consequently, when a welder grips a welding electrode and places it into the jaws of the electrode holder, he does so at considerable risk of electrical shock, for it is at this time that the open circuit (no load) voltage exists at the electrode holder.
To commence welding, a welder places an electrode into an electrode holder, places his welding helmet into position over his face and flicks the electrode against the work piece to establish an arc, and then proceeds to move the electrode along just a fraction of an inch above the work piece to effect welding. As welding progresses, the electrode is gradually consumed in two or three minutes. Then, the stub of the old electrode must be removed and a new electrode inserted into the electrode holder which has on it a live circuit of 70 to 100 volts and the potential of an extremely high current. Since the welding electrode is consumed in less than three minutes, the average production welder will change electrodes more than 100 times per day. Each change of electrode necessitates numerous hand contacts, typically through a leather glove, which, when dry, is a poor insulator. However, the heat of the welding arc and the molten metals present in all electric welding will generally cause a welder's gloves to be damp with perspiration; consequently, the little insulating effect of the leather gloves is totally lost.
Typically, in the construction of ships, bridges, buildings, and in most heavy steel plate fabricating facilities, it is common practice for a welder to be standing, sitting, or kneeling on, or in some other way making bodily contact with the large work pieces involved. It is almost impossible for a welder to insulate himself from the work piece; and when he becomes damp from perspiration, the welder's conductance is increased, thus increasing his potential for electrical shock when he is changing electrodes. As a result, welders are frequently shocked. It is a well documented fact that many welders have been fatally shocked, and others have suffered severe and disabling non-fatal shocks.
This problem has existed from the beginning of electric welding, and it would seem that a practical solution would have been found before now. However, judging by the expanding volume of welding being done without safeguards against shock, it must be assumed that a practical solution has not previously been provided.
In some welding shop operations where a welder is operating in close proximity to a welding power source (15 to 20 feet), he could turn off the power before changing electrodes, making it a safe procedure. Of course, he would them have to return to the power source to turn the power on again before he could proceed to weld. It is obvious that this would more than double the welding time of any job, and, in general practice, it is very seldom done. In the fabrication of large structures such as buildings, bridges, and ships where there are long welding lines (often over 200 feet) running between the power source and a welder, it would be completely impractical and cost prohibitive for a welder to go back and forth to turn the welding power source off and on when changing electrodes.
This is not to imply that nothing has been done to control the problem of shock to a welder. There have been a number of systems designed to modify the welding power source, but their deficiencies are such as to prevent their general acceptance. One such system, perhaps the most popular one, is in the form of a specially designed power source having a control circuit coupled to both the primary and secondary windings of a welding current transformer. An open welding current circuit (caused by breaking an arc) results in contacts in the primary of the welding current transformer being opened and a control voltage of approximately 30 volts to be placed on the secondary winding of the transformer and thus on the welding current cables. A welder now has approximately a 30-volt potential on the electrode to contend with rather than a 70- to 100-volt potential while changing electrodes. Of course, even this smaller potential can cause a shock. Aside from this, the basic problem with this system is that to reestablish the higher open circuit voltage on the welding electrode, it is necessary to ground the welding electrode for a brief instant to complete a control circuit which will operate a primary winding contactor to close it and reapply full power. Typically, power is reapplied after a delay of one to two seconds after a successful grounding of the welding electrode to effect switching.
A further problem with this system, in addition to the fact that there remains a 30-volt potential on the electrode after an arc is broken, is that a welder cannot readily tell when he has succeeded in accomplishing a good ground contact to effect the desired switching, and often several efforts are required to accomplish this. This follows since there may be rust, paint, or scale on the work piece surface, and a good contact is often difficult. As a result of the uncertainty, welding power may come on when unexpected, causing the electrode to stick, or a welder may strike an arc where he does not intend to weld. Further, by virtue of this circuit arrangement, any accidental contact between the electrode and the work piece will cause a welding potential to be applied to the electrode, making it necessary for a welder to always insulate the electrode from the work piece any time that the electrode holder is laid down.
The net result is that while equipment operating as described in the preceding paragraph will provide a measure of protection for a welder, it, and similar equipment, have deficiencies which have prevented widespread use; and, unfortunately, today most welding is done without a welder having any control over the applicaton of power to his welding circuit. Thus, thousands of welders operate in a manner which makes them quite vulnerable to severe shock.
Another problem which arises from this lack of control by a welder over the presence of a welding potential and current is that of accurately striking an arc with the end of an electrode which is sticking out an additional 9 to 18 inches from the electrode holder held in his hand. Since an arc is struck with a welder's hood down over his face for viewing welding through a very dark glass, he does not see where an arc is struck until after it is established. Accordingly, many arc strikes are considerably off target, and a great deal of work is ruined or requires costly repairs because of this.
A further problem is the added shock hazard inherent in alternating current welding systems, which are otherwise less costly to own and operate. It is widely accepted that if the shock hazard from AC electric welding could be reduced, this mode of welding would be significantly expanded.
In view of the foregoing problems, and as a welder and a designer of welding systems for many years, the inventor has determined a control system which does eliminate shock hazards for welders in an absolute and essentially foolproof manner. The control system also provides means for eliminating off-target arc strikes, thus saving industry thousands of dollars in valuable material and rework labor; and all of these benefits are obtained without the requirement for any remote switching function extending between a welder and the power source, and without any modifications to the power source, which may be either AC or DC.